Cavity resonator circuit



Ap 30, 1957 R. L. MEISENHEIMER 2,790,855

CAVITY RESONATOR CIRCUIT Filed April 17, 1953 2 Sheets-Sheet 1 INVENTORY fAyMoA/pl. Mum/15mm LE v firm/Fur. I t

ATTORNEY April 30, 1957 R; L. MEISENHEIMER CAVITY RESONATOR CIRCUIT Filed April 17, 1953 2 Shets-Sheet 2 z 1/ N! Z N w L w m K BY I ATTORNEY 2,790,855 v Patented Apr ls 0, 1957 CAVITY RESONATOR CIRCUIT Raymond L. Meisenheimer, Haddonfield, N. .L, assignor to Radio Corporation of America, a corporation of Delaware Application April 17, 1953, Serial No. 349,433

11 Claims. (Cl. 179-171) This invention relates generally to electron discharge device circuits employing cavity resonators in association therewith, and particularly to such circuits which utilize multigrid electron discharge devices such as tetrodes.

'Tetrode amplifier devices offer certain advantages when employed together with cavity resonators as tuned circuits, but to secure true tetrode operation, both the cathode and the screen grid of the tetrode must be at the same potential with respect to the radio frequency signal. Furthermore, the input cavity resonator must be tuned to resonance at the desired radio frequency, and energy fed back from output to input must be neutralized. The satisfaction'of these requirements has involved difficulties which have limited the eflicient use of such multigrid tube circuits at frequencies above 100 megacycles.

This invention has as an object the true neutralization of tetrode and other multigrid electron discharge devices in association-with cavity resonators as tuned circuit elements by providing an adjustable screen grid series im pedance to isolate the input and output circuits at the operating frequency.

Another object of this invention is to provide a mechanically convenient adjustable neutralization arrangement for multigrid electron discharge devices utilizing cavity resonatorsas tuned circuits.

A further object of this invention is to facilitate the tuning and neutralization of a multigrid tube and its associated cavity resonator in a circuit adapted for operation in the very-high and ultra-high-frequency ranges.

Still another object of the invention is to provide a cavity resonator circuit for modulated power amplifiers wherein the control grid circuit of the amplifier offers very low input capacitive loading and hence small shunting action to a modulating signal applied thereto.

In accordance with the invention, these desirable objects are achieved by the use of a cavity resonator structure which includes means for adjusting the series screen grid mutual impedance seen by the input and output circuits of the discharge device. The'invention is explained with reference to a folded coaxial line cavity resonator in association with a tetrode electron discharge device. The arrangement and construction of the foldedcoaxial line resonatoris such that an outer cylindrical wall electrically contacts the screen grid terminal of the tetrode, an intermediate cylindrical wall contacts the control grid electrode terminal and an inner cylindrical wall is electrically coupled to the cathode terminals. The inner and'outer cylindrical walls are shortcircuited, and the intermediate cylindrical wall is adjustable in length. The overall length of the folded coaxial line from the cathode electrode to the screen grid electrode is effectively one wavelength (or an integral multiple of one wavelength) at the operating frequency. The folded line may be thought of as a two conductor line one wavelength long, one end of which is connected between the cathode andthe control grid and the other end of which is connected between the screen grid and control grid.

The outer cylindrical wall which is connected to the screen grid terminal of the tetrode tube has a portion adacent to the screen grid terminal which is slotted to provide a mutual inductance element. The amount of the 1nductance offered by these slots to the flow of radio fre quency energy is made adjustable by providing bridging l-mks which, by virtue of their position and number, act to alter the series screen grid impedance seen by the input and output circuits of the tube. This inductance in series with the screen grid terminal of the multigrid tube is used to adjust the net mutual impedance between the input and output circuits to the value required for neutralization of the voltages fed back from output to input by the interelectrode capacitance.

A more detailed description of the invention follows in connection with the accompanying drawings, wherein:

Figure 1 is a cross-section in elevation of a tetrode electron discharge device power amplifier system embodying the invention; and' Figure 2 is a plan view of a grid terminal contacting ring embodying the adjustable inductance element utilized in the invention.

Referring to Figure 1, there is shown an electron discharge device or vacuum tube 11 having an anode terminal 13, a screen grid terminal 15, a control grid terminal 17, and filamentary cathode terminals 19., A cavity resonator structure includes input and output circuits for the discharge device 11. The input cavity structure consists of an outer metallic cylindrical wall 21 which is mechanically secured to a screen grid terminal contacting ring 23, an inner metallic cylindrical wall 25' which is electrically coupled to the filamentary cathode through a pair of capacitors 27, and a cylindrical metallic intermediate wall or grid bell 29 coupled to the control grid terminal 17. The cylindrical grid bell 29 is mechanically suspended from the inner wall 25 but is electrically insulated therefrom. The effective length of the grid bell 29 is adjustable by means of an auxiliary cylindrical metallic grid bell 31 which is maintained in electrical contact but in slidable mechanical engagement with the grid bell 29 by springs 33 on the ends of both the grid bell 29 and the auxiliary bell 31. a

Radio frequency energy is supplied to the input cavity through an input coaxial transmission line which includes the inner cylindrical wall'25 as its outer conductor over a portion of its length, a further cylindrical tube 35 as an extension of the cylindrical wall 25, and three successive tubes 37, 39 and 41 electrically connected together as the coaxial inner conductor. A metallic cup 42 surrounding the filamentary cathode terminals 19 forms one side of the radio frequency input line while the inner cylindrical wall 25 forms the other side. The radio frequency energy is therefore, introduced in the cathode-control grid portion of the input cavity between the metallic cathode cup 42 v and the inner cylindrical wall 25.

The input cavity is closed at the end remote from the electron discharge device 11 by a slidable metallic annular disc 43 having contacting springs 44 which engage the inner cylindrical wall 25 and the outer cylindrical Wall 21. The metallic annular closing disc 43 carries thereon rods 46 of insulating material for adjusting the position of the auxiliary grid bell 31 which determines the total length of the grid bell structure 29, 31.

An output cavity resonator is coaxial with and surrounds a portion of the input cavity resonator. The outer wall 21 of the input cavity serves as the inner cylindrical wall of the output cavity, and a cylindrical metallic exterior anode wall 45 serves as the outer conductor. The coaxial output cavity resonator is tuned to resonance by means of an adjustable annular shorting bar 47 bridging the walls 21 and 45. p v

The amplified radio frequency energy is extracted fromthe-outputleavityby a coupling loop 49 connected to I an output coaxial transmission line 50 which extends through an opening in the exterior anode cavity wall 45. The output coupling loop 49 is electrically returned to the anode cavity wall by a stud clamp 51. The stud 51 is slidahle toward and away from the other end of the output coupling loop 49 to vary the length of the loop and thereby to change the inductance thereof. A loop shunt 52 is also included which extends from the inner conductor of the coaxial line to actually short out a portion of the loop. The loop shunt member 52 may be removed so that two ranges of inductance of the output coupling loop 49 are obtainable. 'Thesegtwo means for changing the inductance of the output coupling 7 loop enable the same mechanical configuration of coupling loop to be used overia very wide frequency band. It should be noted that although the output cavity is shown as being coaxial with and surrounding a portion of the input cavity,'it may be more convenient for a certain design to make'the output cavity in a hat disc shape, or a coaxial or waveguide cavity=extending away from the input cavity, or other known configuration. The tuning system of the input cavity resonator arrangement of the present invention is operable with each of these types of output cavity resonators.

Anode operating potential is supplied to the anode terminal of the electron discharge device 11 through an anode supply lead 57 extending to the positive terminal B+ of a source of unidirectional potential. The electron discharge device 11 is supported in the cavity'resonator structure by an anode retaining shoulder 53. The shoulder 53 is mechanically joined to but-electrically insulated from a top cover plate 55, which is mechanically and electrically secured to the exterior anode wall 45.. A

sheet of dielectric material 61 electrically insulates the anode retaining shoulder 53 from the top cover plate 55. A crimp -or corrugated roll 63 is used to support the anode retaining shoulder 53 and the weight of the electron discharge device 11. This structure, including the anode retaining shoulder 53, the top cover plate 55, and the dielectric material 61, forms an anode direct current isolation capacitor between the anode retaining shoulder 53 and the top cover plate 55.

Filament heating energy is supplied to the filamentary cathode terminals 19 through a pair of filament leads 69 which are directly connected through spring clamps 70 which engage the filamentary cathode'terminals 19. It is convenient to bring the filament leads '69 out of the cavity resonator structure on the inside of'the radio frequency (R. F.) input line, that is, through'the center of the tubular conductors 37, 39, 4'1. Withthis arrangement, the filament leads "69 do not interfere with any of the other electrical connections.

Grid biasing and modulating signal potential to be impressed on the control grid terminal 17 may be supplied through a grid cable 71. The grid cable 71is shown as having a coaxial surrounding conductor 72 which functions as a quarter wave choke at the frequency of operation of the input cavity. The grid cable 71 is connected to a terminal 73 on the grid bell structure 29. A directand alternating current path to the control grid terminal 17,is then available through the grid cable 71, the terminal 73, the gridbell structure 29 and the control grid contacting ring 74.

In operating tetrode electron discharge devices as grid modulated high power" amplifiers in the very-highfrequency and ultrahigh-frequency ranges, two conditions must be fulfilled: (1-) the input cavity must be resonant at the operating frequency, and (2) both the cathode and screen grid of the tetrode'must be at approximately. the same potential with respect to 'the radio frequency signal.

Considering the input cavity circuit of the present invention, the grid bell structure 29, 31 divides the input line cavity into two parts 2, a cathode-to-control ,grid por tion and a control grid-to-screen portion. The cathodeto-control grid portion is a section of coaxial line in which the tubular inner wall 25 coupled to the cathode terminals 19 is the inner conductor and the inner surface of the grid bell 29 and auxiliary bell 31 is the outer conductor. The control grid-to-screen grid portion is also a section of line, coaxial with and surrounding the cathode-to-control grid portion, in which the outer surface of the grid bell 29 and auxiliary bell 31 serves as the inner condutcor and the outer cylindrical wall 21 coupled to the screen grid is the outer conductor. The input cavity may be considered as a full wavelength transmission line with one portion folded back over the other so that one common wall (the grid bell 29 and auxiliary bell 31) serves as the outer conductor of one line and as the inner conductor of the other line. The input circuit is tuned to resonance by adjusting the total efiective length ofthe grid bell 29 and the auxiliary bell 31. The input circuit then operates as a resonant full wavelength line having one end connected between cathode and control grid and the other end connected between control grid and screen grid. It should be borne in mind that in the very-high and ultrahigh-frequency ranges in which it is contemplated the invention will find its primary application, the portion of the full wavelength line which is exterior to the actual electron discharge device elements will be shorter than a full wavelength because the length of the discharge device elements themselves is an appreciable fraction of a wavelength, and also because of the shortening etfect of the 'interelectrode and other capacitances between the line conductors.

, In attempting to tune an input coaxial cavity resonator by adjusting the length of the grid bell 29 and auxiliary bell 31, it is found that when resonance of the input circuit is attained, most often the voltages existing between the cathode-and control grid and screen grid are not the same. In other Words, even though the input line is made resonant at one wavelength, difiiercnt instantaneous radio frequency voltages appear across the two ends. This means that the screen grid of the electron discharge device will have a radio frequency potential different from the cathode. The presence of a radio frequency potential difference between the screen grid and cathode may have a degenerative effect on the power gain of the amplifier.

This voltage difference at the two ends of the input cavity resonant line results from the fact that the interelectrode capacitance of the electron discharge device is not the same between the control grid and cathode as the capacitance between the control grid and screen grid, due to the necessary ditference in geometric configuration of the elements of the discharge devices. Also, the external connections and diameters of the line sections affect the voltages at the ends of the line.

The impedance of the two ends of the full wavelength coaxiallineforming the input cavity maybe made to match approximately the interelectrode impedance of the vacuum tube elements to which each end of the line is connected by a proper proportioning of the ratios of the diameters of the inner and outer conductors of the line sections. This approximate matching may be done in the design of the particular cavity resonator to operate in conjunction with a certain type of electron discharge device.

However, even when the same instantaneous radio frequency voltages exist across the two ends of the input line, the need for neutralization of the energy fed back within the tube from the output to the input exists, and becomes more and more important as higher frequencies of operation are employed.

This invention enables multigrid electron discharge devices used in conjunction with cavity resonators as tuned circuits to be neutralized by means of an adjustable series inductance in the portion of the resonator adjacent to the screen grid terminal. ,This inductance is in the form of a plurality of slots in the grid terminal plate of the outer cylindrical wall 21.

Figure 2 shows a plan view of a closing plate for the outer cylindrical wall 21 which acts as the screen grid terminal plate 75. This screen grid terminal plate 75 is shown as having a plurality of slots 77 which are arcuately formed and of elongated configuration spaced around the screen grid terminal plate 75. The inductance oifered by the arcuate slots 77 is mutual to both the input and output cavities, since the outer cylindrical wall 21 and the screen grid terminal plate 75 form a common wall member between the two cavities. The inductance oifered by the arcuate slots 77 is made adjustable =by providing a plurality of bridging links 79. These bridging links 79 provide parallel inductance paths for the radio frequency energy across the face of the screen grid terminal plate 75. When no bridging links 79 are used, a maximum mutual inductance is interposed in series with the screen grid terminal, which inductance is, as stated, mutual to both input and output. One bridging link 79 in each arcuate slot may be used to reduce this series screen grid inductance, and a very fine adjustment of the value of the inductance is obtained by moving the bridging link 79 circumferentially in the arcuate slot 77. The inductance may be further decreased by providing two or more bridging links 79 for each arcuate slot 77. The minimum inductance is obtained for any given number of bridging links by spacing them so that a number of paths are equally spaced circumferentially throughout the length of the are occupied by the slots 77.

The series inductance in the screen grid terminal plate is adjusted for a particular circuit condition by choosing the proper number and position of the bridging links 79 to give the proper value of net mutual impedance between the input and output circuits for true neutralization of the voltages fed back from the output to the input circuits by the interelectrode capacitance. Part of this mutual impedance between the input and output circuits is formed by a capacitive coupling between the outer cylindrical wall 21 and an electrical extension 21 of the outer cylindrical wall which forms the cylinder actually closed by the screen grid terminal plate 75. The outer cylindrical Wall 21 and its extension 21 are maintained in direct current isolation by a sheet of dielectric material 81 between the wall surfaces 21 and 21'.

If the mutual impedance between the input and output circuits is of the proper value both as to magnitude and phase, the instantaneous radio frequency volt-age fed back through the series screen impedance exactly neutralizes that impressed from output to input through the anode-control grid capacitance. True neutralization is, therefore, achieved and true tetrode operation of the vacuum tube 11 is obtained. This has the result of improved stability of operation, increased power efliciency, and allows a given vacuum tube to be operated in the ideal manner at higher radio frequencies. The embodiment of the invention described is particularly useful in a system for transmitting television signals, both in the aural and visual circuits.

The circuit arrangement of this invention achieves true neutralization with a very simple mechanical adjustment for use at very-high and ultra-high frequencies. This adjustment of the mutual screen grid series impedance is accomplished in practice with a negligible detuning effect on the resonant frequency to which the input and output circuits are tuned.

What is claimed is:

1. An electron discharge device system for a multigrid electron discharge device having at least a cathode, a control grid, and a screen grid, comprising a tunable coaxial resonator input circuit, said input circuit including a folded coaxial line, one end of said line having one conductor adapted to be coupled to said screen grid and the other conductor adapted to *be coupled to said con trol gridand the other end of said line having said one conductor adapted to be coupled to said cathode and said other conductor adapted to be coupled to said control grid, means for changing the length of said line, a toroidal disc adapted for coupling said screen grid to said one conductor at said one end of said line, said disc having an arcuate slot therein, and a metallic bridging link contacting opposite sides of said 'slot and being adjustable in position in said slot.

2. An electron discharge device system for a multigrid electron discharge device having at least a cathode, a control grid, and a screen grid, comprising a tunable coaxial cavity resonator input circuit, said input circuit including afolded coaxial line, one end of said line having one conductor adapted to be coupled to said screen grid and the other conductor adapted to be coupled to said control grid and the other end of said line having said one conductor adapted to be coupled to said cathode and said other conductor adapted to be coupled to said control grid, means for changing the length of said line, a'toroidal disc adapted for coupling said screen grid to said one conductor at said one end of said line, said disc having a plurality of arcuate slots therein, and a metallic bridging link for each of said slots contacting opposite sides of said slot and being adjustable in position in said slot.

3. A radio frequency system comprising a multigrid electron discharge device having a cathode, and first and second grids, a folded coaxial line resonator having three coaxially arranged sections of metallic tubing, means respectively coupling said sections of tubing to said cathode and first and second grids, the axis of said coaxial line resonator being coincident with the axis of said discharge device, the outer two sections of said tubing of said resonator being longer than the intermediate section,

said outer two sections of tubing being electrically con nected together at their ends farthest removed from said discharge device, said means coupling said second grid to said section of tubing comprising a plate having an aperture therein contacting the terminal of said second grid, an arcuate slot in said plate in proximity to said aperture, and a metallic bridging link contacting opposite sides of said slot and being adjustable in position in said slot.

4. A radio frequency system comprising a multi-grid electron discharge device having a cathode, and first and second grids, a folded coaxial line resonator having three coaxially arranged sections of metallic tubing, means respectively coupling said sections of tubing to said cathode a and first and second grids, the axis of said coaxial line resonator being coincident with the axis of said discharge device, the outer two sections of said tubing of said resonator being longer than the intermediate section, said outer two sections of tubing being electrically connected together at their ends farthest removed from said discharge device, said means coupling said second grid to said section of tubing comprising a plate having an aperture therein contacting the terminal of said second grid, a plurality of arcuate slots in said plate in proximity to said aperture, and a metallic bridging link for each of said slots contacting opposite sides of said slot and being adjustable in position in said slot. i

5. An electron discharge device system for a multigrid electron discharge device having an anode, a cathode, a control grid, and a screen grid, comprising, an output cavity adapted to be coupled to said anode, a tunable input cavity, said input cavity including a folded coaxial line, one end of said line having one conductor adapted to be coupled to said screen grid and the other conductor adapted to be coupled to said control grid, and the other end of said line having one conductor adapted to be coupled to said cathode and said other conductor adapted to be coupled to said control grid, means for changing the physical length of said line, a toroidal disc coupling 7 forming a common wall between said input cavity and said output cavity, said disc having an arcuate slot therein, and a metallic bridging link contacting opposite sides of said slot and being adjustable in position in said slot.

6. A system as defined in claim 5, wherein: said arcuate toroidal disc is provided with a plurality of arcuate slots therein, and a metallic bridging link for each of said slots contacting opposite sides of. said slots and being adjustable in position therein.

7. A radio frequency system comprising a mu-lti-grid electron discharge device having an anode, a cathode, and tirstand second grids, a folded coaxial line resonator having three coaxially arranged sections of metallic tubing, means respectively coupling said sections of tubing to said cathode and firstand second grids, the axis of said coaxial line resonator being coincident with the axis of said discharge device, the inner and outer sections of said tubing being longer than the intermediate section, said inner and outer sections of tubing being connected together at their ends farthest removed from said discharge device, said means coupling said second grid to said section of tubing comprising a plate having an aperture therein contacting the terminal of said second grid, an arcuate slot in said plate in proximity to said aperture, a metallic bridging link contacting opposite sides of said slot and being adjustable in position in said slot,

and anoutput cavity coupled to'said anode and having a common wall with said coaxial line resonator including said slotted plate.

8. In combination, a vacuum tube having input and output electrodes including a screen grid electrode, concentric coaxial line input and output cavities coupled to said electrodes and having a fiat circular common wall therebetween coupled to and in close proximity with said screen grid electrode, said common wall being apertured to receive said vacuum tube and having an arcuate slot therein, and a metallic bridging link connecting opposite sides' of said slot and being adjustable in position in said slot.

9. In combination, a vacuum tube having input and output electrodes, input and output cavity resonators coupled to said electrodes and having a flat circular common wall therebetween, said wall being apertured to receive said vacuum tube and having an arcuate slot therein, and a pair of bridging links each connecting opposite sides of said slot. and being adjustable in position along said slot, said links being on opposite sides of the center of said slot.

10. In combination, a vacuum tube having input and output electrodes, input and output cavity resonators coupled to said electrodes and having a fiat circular com mon wall therebetween, said wall being apertured to receive said vacuum tube and having an elongated slot therein, and a pair of bridging links extending across the narrower dimension of said slot and connecting opposite sides of said slot and being adjustable in position along said slot, said links being on opposite sides of the center of said slot.

11. In combination, a vacuum tube having input and output electrodes, input and output cavity resonators,

coupled to said electrodes and having a common wall therebetween, said wall being apertured to receive said vacuum tube and having a slot therein, said slot being longer than it is wide, and a conductive bridging link connecting opposite sides of the wider dimension of said slot, said link being adjustable in position along the length of said slot and including means to clamp said link to the sides of said slot.

References Cited in the file of this patent UNITED STATES PATENTS Miyaji Jan. 17, 

